Add warning commentary making important assumptions explicit.

1 files changed, 8 insertions, 0 deletions

diff --git a/generic/tclCmdIL.c b/generic/tclCmdIL.c
index 0a1b4fe..9fbb0ad 100644
--- a/generic/tclCmdIL.c
+++ b/generic/tclCmdIL.c
@@ -2179,6 +2179,14 @@ Tcl_JoinObjCmd(
     resObjPtr = Tcl_NewObj();
     for (i = 0;  i < listLen;  i++) {
 	if (i > 0) {
+
+	    /*
+	     * NOTE: This code is relying on Tcl_AppendObjToObj() **NOT**
+	     * to shimmer joinObjPtr.  If it did, then the case where
+	     * objv[1] and objv[2] are the same value would not be safe.
+	     * Accessing elemPtrs would crash.
+	     */
+
 	    Tcl_AppendObjToObj(resObjPtr, joinObjPtr);
 	}
 	Tcl_AppendObjToObj(resObjPtr, elemPtrs[i]);